[hf] fixup mr !599 - cleanup SignalReceiver/ContactStore

cleans up !599 (merged)

SignalReceiver:

• make helpers in different branches of #dispatch return Unit instead of Job? (we never used the job and it was odd to return null from 3/4 of the branches!)
• call ContactStore#create with uuid and phone number from #handlePreKeyBundle (instead of calling overloaded #create that takes an envelope as arg) to (1) be simpler, (2) make it more explicit why we are storing both pieces of data at this point in the session lifecycle
• revise the comment for #handlePreKeyBundle (it is a "hot spot" and likely to be the source of lots of confusion to future us when less debugging state is loaded in our heads!)

ContactStore

• revise ContactStore#hasContact to be slightly more straightforward (if less terse)
• remove overloaded #create (which is no longer called)